1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting diode fabricated on a light transmissive and conductive substrate, such as a GaN substrate, and more specifically to a semiconductor light-emitting diode that can achieve a high light-emitting efficiency by uniformly supplying a current to a light-emitting layer and raising the light extraction efficiency.
2. Background Art
In recent years, the research and development of blue, white or ultraviolet semiconductor light-emitting diodes (LEDs) using a nitride-containing III-V group compound semiconductor, such as AlInGaN, have been carried out, and already in practical use (for example, refer to Japanese Patent Laid-Open No. 2005-166840). The mainstream of present semiconductor light-emitting diodes is of a type wherein the crystals of a nitride semiconductor are grown on a sapphire substrate because of the low costs thereof.
However, since there is a large lattice mismatch between the sapphire substrate and the crystals of a nitride semiconductor grown on the substrate, when the nitride semiconductor is directly grown on the sapphire substrate, a very large number of threading dislocations having a density of 109 to 1010 cm−3 or higher are present in the light-emitting layer. Since these threading dislocations produce non-light-emitting recombination centers of carriers in the light-emitting layer, the light-emitting efficiency is lowered.
Therefore, a method for reducing threading dislocations by growing a buffer layer at a low temperature on a sapphire substrate has been used. However, even if this method is used, the density of threading dislocations is not so small, and is about 107 cm−3. On the other hand, the density of threading dislocations of presently marketed GaN substrate is about 105 cm−3, and the further reduction of the density of threading dislocations is expected in the future. Therefore, to improve light-emitting efficiency, the use of GaN substrates is highly effective.
As the methods to mounting a semiconductor light-emitting diode of the type wherein a p-type electrode and an n-type electrode are formed on the principal surface and the rear surface, respectively, the following two methods can be used. One is a method wherein die bonding is performed with the substrate side, i.e., the n-electrode side down and light is extracted from the principal-surface side. On the contrary, the other is a method wherein die bonding is performed with the principal-surface side, i.e., the p-electrode side down and light is extracted from the rear-surface side.
FIG. 6 is a sectional view showing a conventional semiconductor light-emitting diode of the type wherein the rear surface of a substrate is die-bonded and light is extracted from the principal surface of the substrate. As FIG. 6 shows, on a substrate 1, as semiconductor layers, an n-type AlGaN clad layer 2 having a thickness of 1.0 μm and an Al composition ratio of 0.07; a light-emitting layer 3 composed of four InGaN barrier layers (not shown) each having a thickness of 7 nm and an In composition ratio of 0.02 and three InGaN well layers each having a thickness of 5 nm and an In composition ratio of 0.10; a p-type AlGaN clad layer 4 having a thickness of 100 nm and an Al composition ratio of 0.07; and a p-type GaN contact layer 5 having a thickness of 20 nm are laminated. On the rear surface of the substrate 1, an n-electrode 6 composed of Ti/Au is formed, and on the GaN contact layer 5, p-electrodes 7 composed of Pd/Au are formed. Openings are provided in the p-electrodes 7, and light is extracted mainly from this portion.
Here, if the distances L between the p-electrodes 7 are excessively longer than the distance t between the p-electrodes 7 and the light-emitting layer 3, the quantity of current supplied to the light-emitting layer 3 becomes uneven, and the regions where current is not supplied are produced to lower the light-emitting efficiency. In order to prevent this, the distances L between the p-electrodes 7 are reduced. Thereby, since there is only a high-resistance thin p-type semiconductor layer between the light-emitting layer 3 and the p-electrodes 7, the current spreading in the lateral direction produced when current flows from the p-electrodes 7 to the light-emitting layer 3 is minimized.
The distance t between the p-electrodes 7 and the light-emitting layer 3 in the conventional semiconductor light-emitting diode is 1 μm or shorter. Consequently, the current spreading in the lateral direction is at largest several micrometers. Therefore, to prevent uneven injection of current the opening width L of the p-electrodes 7 must be several micrometers or less.
However, when the opening width L of the electrodes 7 was reduced, as Fig.7 shows, there was a problem wherein a part of the light generated from the light-emitting layer was reflected or absorbed by the p-electrode 7 causing the lowering of the light emitting efficiency.
To solve this problem, as FIG. 8 shows, there is a method to reduce the reflection and absorption of light by using a light transmissive electrode 8 as the p-electrode. However, even in this case, the light is reflected and absorbed, and when the light emitting wavelength of a semiconductor light-emitting diode is short, a serious problem arises. Also when the electrode is thin, since the resistance of the electrode elevates, the quantity of current lowers to the location far from the wire for current supply, and the uniform current supply is difficult.
Furthermore, as FIG. 9 shows, there is a method to extract light from the rear surface by forming an n-electrode 6 on the portion from which a part of the semiconductor layer has been etched of f and mounting with the principal surface side facing down. However, since the n-electrode 6 becomes apart from the p-electrode 7, it is difficult to uniformly supplying the current. In addition, since a part of the light-emitting layer 3 is etched off, the area of the light emitting region is reduced. Furthermore, since the distance between the n-electrode 6 and the p-electrode 7 is shortened, there is a problem wherein it is difficult to die-bond with both electrodes insulated.